The present invention relates to a method for preparing synthetic powders containing emulsifiers and endowed with good paste properties. These powders are prepared by spray-drying polymer dispersions by means of binary material nozzles and with a spray ratio of 1.6-5.0 kg. of spray air to each kg. of dispersion. The spray-dried powder is then treated with an air jet. The synthetic powders so obtained are particularly suitable for advantageous use in the preparation of platisols.
The state of the art of vinyl chloride polymer and copolymer emulsions useful in the present invention may be ascertained by reference to U.S. Pat. Nos. 3,627,717 and 3,732,178, and the Kirk-Othmer "Encyclopedia of Chemical Technology," 2nd Edition, Vol. 21, (1970) pages 369-412, particularly pages 373-374 where Emulsion Polymerization is disclosed, and pages 403-404 where Dispersion Resins which include plastisols and plastigels are disclosed. Kirk-Othmer states that plastigels are plastisols having high viscosities and yield values and that putty-like pastes may be prepared therefrom. The state of the art of spray drying, pneumatic atomizers and polyvinyl chloride polymer and copolymer powders produced therewith may be ascertained by reference to U.S. Pat. No. 3,805,869 and Kirk-Othmer, Vol. 7 (1965), pages 360-368 under the section entitled "Dispersion-Type Direct Dryers,"0 particularly pages 362-364 wherein spray dryers and pneumatic atomizers are disclosed. U.S. Pat. No. 3,805,869 discloses the preparation of uniform, finely divided powdery vinyl chloride polymer particles containing less than 2 weight percent particles having a diameter greater than 40 microns obtained by spray drying an aqueous dispersion of the polymer and a carrier gas through a two-fluid atomizer.
As disclosed by the state of the art, plastisols are dispersions of vinyl resins in plasticizer.
They are used in making molded bodies, for instance by using the centrifugal casting method, and for making coatings or layered materials using a painting method. Following molding, the plastisols are solidified by hot gelling.
A number of requirements must be met with regard to the properties of the plastisols and those of the powders suitable for plastisol preparation. These include:
a. The plastisols must remain intermixed, so that constant products with smooth surfaces are made at low cost. Hence special fineness of the synthetic powder to be dispersed is demanded. Generally such a powder should contain only slight amounts, such as less than 2 weight percent, exceeding 40 microns in diameter.
b. Furthermore, the pastes should be endowed with as flat a viscosity curve (viscosity dependence on the gradient of shear) as possible. Also, the level of viscosity curve should be as low as possible. These properties are especially significant with respect to the frequently applied process of rotational casting when making imitation leather or foam imitation leather.
c. Also, high bulk density is required of the powders used for paste preparation. This high density assumes good powder fluidity and causes good storage behavior while being transported.
d. Lastly, the powders should lend themselves to good pasting, in other words, the energy required when homogenizing powder and plasticizers should be as little as possible. Pasting behavior may be expressed in terms of the maximum energy required during homogenization, which must take place at a constant rate, and/or in terms of the time after which a certain amount of power is expended.
The industrially preferred method for the preparation of powders suitable for making plastisols is the spray-drying of polymer dispersions.
In this regard, the PVC dispersion may be sprayed by means of a two-fluid atomizer into a drying tower. The required hot air is introduced into the tower from the top. The dried product is obtained at the lower end of the tower. According to the related state of the art, as disclosed in U.S. Pat. No. 3,883,494, the spray ratio is 1.6-5.0 kg. of sprayed air to one kg. of dispersion. According to the method of U.S. Pat. No. 3,883,494 uniform, finely divided powdery vinyl chloride particles containing less than 2 percent by weight particles having a diameter greater than 40 microns are obtained.
This means that the powders ab initio are obtained in a fineness suitable for paste preparation. Hence they no longer require otherwise required air sifting. Sifting only occurs for removing any impurties that might be present and these impurities result from wall cakings of the spray tower. The powders prepared in conformity with the method of U.S. Pat. No. 3,805,869 evidence high bulk densities and good rheological behavior, but however, their pasting behavior needs to be improved.